A network operator who wishes to introduce a broadband service, for instance xDSL (Digital Subscriber line), must be able to measure certain parameters for a wire pair that is to be used to deliver the service. This is necessary both to ensure that the service can be successfully provided and to enable the network operator to guarantee service quality. There are many advantages if the measurements can be performed on a two sided basis. This means that a signal source capable of transmitting test messages/signals, upon request, must be placed at the customer's end of the line. The results of applying a test message/signal to the line at the customer's end are measured at the CO (Central Office) end. The present invention provides an active POTS splitter (“Plain Old Telephony Service” splitter) which includes this testing functionality.
When delivering a broadband service, such as xDSL, without inband POTS, it is necessary to separate the analog POTS signal and the xDSL signal from each other at both the CO (Central Office) and the CP (Customer's Premises). This can be achieved by using analog passive high pass and low pass fitters. FIG. 1, the accompanying drawing, illustrates this principle.
However, a passive POTS splitter design lacks the performance of an active splitter design, see J. Cook and P. Sheppard, “ADSL and VADSL Splitter Design and Telephony Performance”, p. 1634, December 1995, IEEE Journal on Selected Areas in Communications, ISSN 07-8716. This results in either: poorer POTS transmission; or if the cutoff frequency of the low pass filter in the splitter is increased, a waste of valuable spectrum that could otherwise be used to support higher data rates in the xDSL modem.
If an active POTS splitter design, with impedance matched circuits, is used, the filter design criteria become more reasonable while, at the same time, the spectrum, or bandwidth, use becomes more efficient, without degradation in the ordinary telephony transmission. The preferred solutions, especially as xDSL becomes, more common, is the active POTS splitter design, or inband POTS (which implements the telephony service as a part of the xDSL data stream).
If the active splitter design is used and the splitter is implemented on a single chip, the present invention proposes the incorporation of test functionality for the line between the CP and the CO, or ONU (Optical Network Unit), on this chip. This enables two-sided measurements on the line, both during installation and during operation. The measurements are performed at the CO end upon request, or when the test device automatically sends a test message signal. In this way there will be no need for field technicians at the CP side. If necessary, these chips can have a unique identity code that is transmitted to the CO each time a test is started, or on receipt of a request from the CO.
The test messages/signals should either be specially designed for a certain measurement case, or general applicable signals which can be used with a range of tests, e.g. pulses, steps, or chirps, to estimate the transfer function of the line. One example of a parameter requiring two-sided measurement is attenuation. The test sequence could, in this case, be a series of sinusoidal signals, with known amplitudes, sent from the test device to the CO, in sequence. The sequence can, for example, comprise 10, 100, or any other number, of tones, starting from either low, or high frequency and then varying in frequency towards the other side of the frequency spectrum. Each tone is transmitted for a relatively long period of time, so that synchronization is not a problem and so that the measurement can be performed within the duration of a tone. The start of a test may occur at a predetermined time interval after a test message request.